1. Field of the Disclosure
The present disclosure relates to a nitride-based light emitting device and a method of manufacturing the same, and more particularly, to a nitride-based light emitting device with a transparent film electrode structure designed to provide a low specific contact resistance and high light transmittance and a method of manufacturing the same.
2. Description of the Related Art
The formation of an ohmic contact between a semiconductor and an electrode is of considerable importance in realizing light emitting devices such as light emitting diodes (LEDs) and laser diodes (LDs) that utilize a nitride-based compound semiconductor such as gallium nitride (GaN).
GaN-based light emitting devices are classified into top-emitting LEDs (TLEDs) and flip-chip LEDs (FCLEDs). In commonly used TLEDs employing a Ni/Au ohmic contact layer, light exits through the ohmic contact layer in contact with a p-cladding layer.
A Ni/Au layer acts as a semi-transparent ohmic contact layer having excellent a specific contact resistance of 10−4 to 10−3 Ωcm2. Annealing of the Ni/Au layer at a temperature of 500 to 600° C. in an oxygen (O2) atmosphere leads to the formation nickel oxide (NiO) at the interface between the p-GaN cladding layer and the Ni layer, thereby lowering a Schottky barrier height (SBH). Thus, holes, which are a majority of the carriers, can be easily injected into the surface of the p-GaN cladding layer, thus increasing effective carrier concentration near the surface of the p-cladding layer.
Annealing of Ni/Au on the p-cladding layer results in disassociation of a Mg—H complex in the GaN, which reactivates Mg dopants by increasing the concentration of Mg dopants on the surface of the GaN. As a result of reactivation, the effective carrier concentration increases above 1019 holes/cm3 on the surface of the p-cladding layer, which causes tunneling conductance between the p-cladding layer and the ohmic contact layer containing NiO, thus obtaining an improved ohmic conductance.
Due to their low light utilization efficiency, a TLED using a Ni/Au semi-transparent film electrode cannot be readily applied to next generation light emitting devices with large capacities and high brightness.
One approach to overcome the limitation in the output power of TELDs is to use indium tin oxide (ITO). ITO is a transparent conductive oxide having superior light transmittance over a semi-transparent Ni/Au used as the conventional p-ohmic contact layer. However, while increasing the output power of a light emitting device, an ITO ohmic contact layer requires a high operating voltage due to a high ohmic contact resistance between p-GaN and ITO, which generates much heat. As an alternative approach, Japanese Laid-open Patent Application No. 2002-164570 discloses that high output power was obtained using p-GaN as a transparent electrode layer. However, since the above-cited patent obtains p-ZnO by codoping Ga and N, it is very difficult to actually use the p-ZnO as a transparent electrode for a p-GaN-based light emitting device. Furthermore, since it is known that the p-ZnO suffers from many reliability problems, its use as an electrode for a p-GaN light emitting device results in a degradation of device reliability.
To address these problems, there is an urgent need to develop a high quality ohmic contact layer with low specific contact resistivity that ensures the formation of a transparent electrode layer.